Fourth version of the onager

Now that I've made a few of these, I decided to make one for a good friend of mine. I made a few changes from the last attempts, including an all wood skein winding mechanism, a square piece of wood for the take-down winch with an eight-tooth ratcheting gear. I also modified the side-plate a little bit so it nails to the side of the upright instead of into the frame rail right below it. And the final change is larger rear cross-piece (half inch instead of three eighths) which I notched out to allow to arm to come farther down.